#b, 1/4" inside hexagonDIN 3126/ISO 1173 D 6.3outer #w, 10 mm for use in tools with 10 mm internal hex drive or internal double hexagon (e.g. "FLEXI" double-ended flexi-joint spanners, T-handle universal joint spanners, box spanners, ratchet ring spanners etc.)internal hex drive with a collar-thrust springbits are easy to insert, lock securely in position and can be removed just as easilyHPQ® high performance steel